Question: In triangle $ABC,$ $AC = BC = 7.$  Let $D$ be a point on $\overline{AB}$ so that $AD = 8$ and $CD = 3.$  Find $BD.$
By the Law of Cosines on triangle $ACD,$
\[\cos \angle ADC = \frac{3^2 + 8^2 - 7^2}{2 \cdot 3 \cdot 8} = \frac{1}{2},\]so $\angle ADC = 60^\circ.$

[asy]
unitsize(0.5 cm);

pair A, B, C, D;

A = (0,0);
B = (13,0);
C = intersectionpoint(arc(A,7,0,180),arc(B,7,0,180));
D = (8,0);

draw(A--B--C--cycle);
draw(C--D);

label("$A$", A, SW);
label("$B$", B, SE);
label("$C$", C, N);
label("$D$", D, S);
label("$8$", (A + D)/2, S);
label("$7$", (A + C)/2, NW);
label("$7$", (B + C)/2, NE);
label("$3$", interp(D,C,1/3), NE);
label("$x$", (B + D)/2, S);
[/asy]

Then $\angle BDC = 120^\circ.$  Let $x = BD.$  Then by the Law of Cosines on triangle $BCD,$
\begin{align*}
49 &= 9 + x^2 - 6x \cos 120^\circ \\
&= x^2 + 3x + 9,
\end{align*}so $x^2 + 3x - 40 = 0.$  This factors as $(x - 5)(x + 8) = 0,$ so $x = \boxed{5}.$